In a code division, multiple access (CDMA) communications system, a plurality of user communication signals can be transmitted within and thus share the same portion of the frequency spectrum. This is accomplished by providing a plurality of different pseudonoise (PN) binary code sequences (e.g., one for each user) that modulate a carrier, thereby "spreading" the spectrum of the resulting waveform. In a given receiver, all of the user signals are received, and one is selected by applying an assigned one of the PN binary code sequences to a correlator to extract only the signal energy intended for the receiver, thereby "despreading" the received CDMA transmission. All other (uncorrelated) user transmissions appear as noise.
In digital spread spectrum communications, data signals, such as voice signals, are digitized (turned into ones and zeros, or the like) and then a pseudonoise (PN) code (also referred to as a signature sequence) is impressed upon the digitized data. A pseudonoise code is usually a high frequency noise-like waveform that is multiplied with the digitized data before it is transmitted; this has the effect of spreading out the spectrum of the signal, hence the term spread spectrum communications. The spread digitized signal is then transmitted to a receiver, at which the same or a corresponding pseudonoise binary code sequence is applied to the received signal to despread it and extract the digitized signal. The digitized signal can then be digital-to-analog converted to obtain the original voice or other data signal. In a multi-user system, if each user in the system uses a different or distinct pseudonoise or signature sequence code, then at the other end of the link, if that particular noise-like waveform or signature sequence is applied to the received signal, the data can then be extracted from that one user's signal, and any other user that is using a different noise-like or pseudonoise sequence will look just like background noise.
There are a number of approaches to accomplish such multi-user channelization. In one such approach, frequency division multiple access (FDMA), each user's transmitter has a distinct band of frequency, and the users do not overlap in frequency. The users can thus be distinguished by tuning to the appropriate frequency band. In time division multiple access (TDMA), every user's transmitter gets a specified time slot; all users then share and utilize the entire bandwidth of the selected channel, but each user transmits for only a short period of time and then turns off to let another user turn on. Thus, in TDMA, there is a series of users that take turns one at a time in a round robin fashion sharing the channel. In code division multiple access (CDMA), all of the users transmit all of the time, and can use the entire frequency band, so that the users can overlap both in frequency and in time in the resulting aggregate signal. In CDMA, the different users are identified or distinguished using pseudonoise codes or signature sequences. Every user is given a distinct pseudonoise code or signature sequence. The aggregate signal is the sum of all of the transmitted signals from all of the users with all of the distinct codes and distinct data. As long as the receiver knows the PN code of the user whose signal he is trying to extract from the aggregate signal that he receives, that receiver can then pull out the signal that it is interested in by knowing the PN code and using that PN code to accomplish despreading. At the modulator for each CDMA user, the signal to be transmitted is spread using the PN code, and then at the demodulator that signal is despread by multiplying the aggregate signal by the same PN code that was used at the transmitter.
Because the subscriber units in an asynchronous CDMA system do not try to coordinate their transmissions, but transmit whenever they want to, the base station will have to align itself with a distinct alignment for each active user's signal. Thus, when the base station is trying to despread each user's signal individually, it will have a different timing offset for every user. On the other hand, in a synchronous CDMA system, signals in the reverse channel (from subscriber unit to base station) are required to arrive with a particular phase alignment. Synchronous CDMA systems are further described in U.S. Pat. No. 5,499,236 issued Mar. 12, 1996 for "Synchronous Multipoint-to-Point CDMA Communication System" by Thomas R. Giallorenzi et al., and in its division U.S. Pat. No. 5,583,853 issued Dec. 10, 1996 for "Synchronous CDMA Transmitter/Receiver" by Thomas R. Giallorenzi et al., each of which is hereby incorporated by reference herein.
A receiver in a code division multiple access (CDMA) communications system having multiple users receives the aggregate signal produced by those users. This aggregate signal is a parallel transmission of a large amount of data from a plurality of users. Signals to and from any particular user are spread with a particular spreading sequence or a pseudonoise (PN) code or signature sequence that are impressed upon the data waveform. The terms spreading sequence, pseudonoise code and signature sequence as used here are intended to be inclusive and synonymous. The pseudonoise code so used is unique for each user in the system. Once each user has its PN code impressed upon its data, the signals from all users are summed together electromagnetically in the atmosphere when they are transmitted from different points in a multipoint-to-point type system. In the case of a point-to-multipoint system, the signals from the users are summed together electronically or digitally in a baseband, such as in the base station of that system, before being transmitted out to the multipoints or subscriber units such as in the cell of a cellular communications system. However, the signals of some multiple users that are so summed together can sometimes each transmit the same or very similar data or repetitive patterns. Thus, notwithstanding use of different spreading codes for the different users, difficulty will be encountered in avoiding interference between those signals. The present invention addresses this problem.
For a point-to-multipoint synchronous CDMA system, the tracking loop at the base station receiver tries to average over multiple data bits in order to average out noise and establish the time of arrival of the signal of choice, reject the other signals and treating them like noise and try to pull the desired signal out from among all of the others in the aggregate signal and track that desired signal. Separating out the desired signal is accomplished by despreading the aggregate received signal. In despreading, the aggregate signal is remultiplied by the spreading code for a particular desired user. Depreading extracts the data from that one channel while rejecting the data from the other channels. Ideally, the other channels would be using PN codes which are orthogonal to the desired user's code, which is being used for despreading. Thus, when the aggregate received signal is multiplied by the PN code of the desired user, and is then integrated over a bit, the signal is despread. Ideally, signals of all of the other users in the system will map to zero when this despreading process is performed. In other words, if the signal of an interfering user is despread using the PN code of the desired user, then in an ideal orthogonal CDMA type environment, the output of the integrator would be zero. In other words, despreading would remove the signal of the undesired user from the received aggregate signal. If the signal of the desired user is present in the aggregate received signal, then that user's data would not be removed by despreading. However, in a real CDMA system, a number of impairments are present which prevent true orthogonality between all of the different spreading codes of the different users, so that there is some small correlation between the different users that would not be removed by despreading. This additional non-zero signal at the output would be spillover from the other channels into the channel for the desired user; this spillover is called multi-user interference, also referred to as multiple access interference or cross-channel interference. This multi-user interference degrades the desired signal produced by despreading. This multi-user interference has certain properties. If the data that the interfering users are sending is random relative to the data transmitted by the desired user, then the multi-user interference has a zero mean. For this reason, if the received despread signal is averaged over a long enough period of time, the multi-user interference will average to zero. Thus, a zero mean for multi-user interference is a desirable feature. However, in the case of correlated data, which is where for some reason multiple users are sending the same data pattern, then the noise from multi-user interference does not have a zero mean. For that it situation, integrating the despread signal does not remove the multi-user interference, but instead will create a skew of the received signal. Typically, there may be additional background noise added to the desired signal. Although this background noise is generally present, the background noise can be removed by integration because its mean should be zero. A data detection function or tracking function will have substantially improved performance if data being transmitted by all users is random with respect to each other. However, in some application such as a fixed wireless loop communications system, there are times where there may be a tendency for the data from several users to be the same. For example, if several users of a telephone system happen to stop speaking at the same time, then the resulting silence pattern transmitted from those users' respective telephones may look the same across users. If this occurs with a large number of users, then tracking performance can be substantially degraded and undesired jitter could then be present.